Dealing with High Soluble Salt Levels in High Tunnels

Some growers have expressed concern about soil test analysis revealing abnormally high soluble salt levels in the soils of their high tunnels. We have seen soluble salt levels increase in the high tunnels at Penn State’s High Tunnel Research and Education Facility, regardless of whether inorganic or organic nutrient sources have been used. In the spring of 2008, levels ranged from 0.37 to 9.38 mmhos/cm.

Nutrient management in high tunnels can be tricky because of the unique environment within them. High tunnels exclude rain and snow which lead to leaching and may result in a build-up of salts that can negatively affect plant growth. In addition, most high tunnels are equipped with drip irrigation, which also limits leaching. Different crops respond differently to soluble salt levels, with some being more sensitive than others.

In a two-year nutrient management study at the Penn State High Tunnel Research and Education facility (Burkhart, 2002). Inorganic fertilizer was applied through drip irrigation lines to supply 75 lbs N, 150 lbs phosphate and 75 lbs of potash per acre per year. Compost was soil incorporated to a depth of 1 ft at rates of 1 or 2 inches in the fall prior to planting. Applying 1 inch of compost supplied 441 lbs of N, 1345 lbs of phosphate and 1559 lbs of potash per acre per year. Applying 2 inches of compost roughly doubled the amount of nutrients added.

Recommended rates of nitrogen, phosphate and potash are 100 lbs per acre for each element showing that excessive nutrients were applied to the soil with compost. The baseline soil soluble salt level was 0.15 mmho/cm. Soluble salt levels remained constant after one year of applying inorganic fertilizer. After two years, they increased to 0.30 mmho/cm. Soluble salt levels tripled to 0.45 mmho/cm after one year and increased over six times to 0.81 mmho/cm after two years when 1 inch of compost was applied. Applying 2 inches of compost resulted in soil salt levels increasing about 5½ times to 0.95 mmho/cm and 13 times to 1.9 mmhos/cm each year of the study, respectively. Applying 2 inches of compost lowered pepper yield which was thought to be related to the high soluble salt levels.

Some ideas for preventing or dealing with high soluble salt levels are:

1. Monitor the soluble salt levels of the soils. By monitoring the soluble salt level of your high tunnel soil, you will know when you need to act.
2. Only place high tunnels in areas with good drainage to promote leaching. If you already have sited your tunnel, it may be too late for this idea. However, when selecting a site for any new high tunnels, choose an area with good soil drainage.
3. Avoid the over application of nutrients. Soluble salt levels can be limited to some extent by applying only the amount of nutrients plants need. Use soil test reports or the Commercial Vegetable Production Recommendations guide to get current recommendations for application rates.
4. Select fertilizers with low salt indexes; limit the use of organic nutrient sources containing animal manures. When possible, select fertilizers with low salt indexes (table follows) to help limit the accumulation of soluble salts. Try to avoid or limit the use of nutrient sources containing animal manures. Animal manures tend to be high in salts.
   Salt Indexes of Various Fertilizers
   Adapted from: Foth & Ellis, Soil Fertility 2nd Ed.
   

   Fertilizer	Salt Index
   Ammonium Nitrate (34-0-0) Sodium Nitrate (16-0-0) Urea (45-0-0) Ammonium Sulfate (21-0-0) Calcium Nitrate Diammonium Phosphate (18-46-0) Monoammonium Phosphate (11-55-0) Superphosphate (0-45-0) Superphosphate (0-20-0) Potassium Chloride (0-0-60) Potassium Nitrate (14-0-47) Potassium Sulfate (0-0-54)	102 100 73 69 65 29 27 10 8 116 74 46

5. Use irrigation water with low salt levels.
6. Use a sprinkler irrigation system to establish seedlings. Seedlings are more sensitive to high soluble salt levels than mature plants. Using sprinkle irrigation can facilitate leaching of salts around the plants.
7. Rotate crops based on salinity tolerances (table will be shown).
8. Leach out salts. As a general guideline for leaching out soluble salts from the top foot of soil, apply 6 inches of water to leach about 50% of the salts, apply 12 inches to leach about 80% of the salts and 24% to leach about 90% of the salts (California Fertilizer Association, Western Fertilizer Handbook, 8th Ed.). Out at the high tunnel facility I’ve been managing 4 high tunnels since 2003. In the fall of 2007 the soluble salt level was on average 0.40 mmho/cm. That November the tops of the tunnels ripped off due to high speed winds and because the plastic was getting old. We decided to leave the tops off until the spring. In April of 2008 we put new tops on and had the soluble salt level of the soils analyzed. On average, the soluble salt level decreased to 0.09 mmho/cm or about 77%. Between November of 2007 and April of 2008 we got about 11.5 inches of rain.

If the tops of the tunnels cannot be removed, leaching soluble salts with irrigation is also an option. This can be accomplished with any irrigation system. The hours required to apply 1 inch of water through a trickle irrigation system depending on the width of the mulched bed is on page C-3 of the 2009 Commercial Vegetable Production Recommendations guide.

In the high tunnels at Penn State’s Center for Plasticulture we use a trickle tape with a 0.40 gpm/100ft flow rate and mulched beds that are 2.5 ft wide. So, we need to run the irrigation system for 6.5 hours to apply 1 inch of water and 78 hours to apply 12 inches of water.

Soil texture (i.e., sand, loamy sand, sandy loam, clay loam, silt loam) is another factor affecting the length of time that an irrigation system needs to be on to apply 1 inch of water. Table C-5 on page C3 of the 2009 Commercial Vegetable Production Recommendations guide lists the maximum number of hours for trickle irrigation systems to apply 1 to 1.5 inches of water based on soil texture.

Elsa Sánchez, Associate Professor of Horticultural Systems Management, Penn State University